The activation of T cells is normally accompanied by several cycles of proliferation, followed by differentiation and the delivery of effector function. Removal of the activated population occurs by apoptosis of the majority of the responding T cells. A currently-prevailing concept is that T cell differentiation, effector function and apoptosis are linked to the process of cell division, such that a certain number of cycles are required before any of these events occurs. However, our preliminary data show that the proliferation and the differentiation of CD4+ T cells can be uncoupled in vivo. In an experimental model in which naive T cells at high precursor frequency are activated by peptide antigen, we have documented that a subset of the cells is activated but arrested in the G1 phase of the cell cycle. Nevertheless these cells express both Interleukin-2 and Interferon-y, suggesting that they have differentiated into Th- 1 effector cells without cell division. This project will determine whether similar differentiation without division is possible in T cells committed to the Th2 (i.e. the IL-4 secreting) fate, and what mechanisms result in cell cycle arrest in these activated cells. Other preliminary data show that, in contrast to effector function, the apoptosis of these CD4+ T cells is closely correlated with proliferation. We will therefore test whether proliferation is required to allow the cells to undergo apoptosis in vivo, and determine which death mechanisms are involved. This analysis of the relationship between cell cycle progression and key cell fate decisions in CD4+ T cells in vivo will reveal basic information about T cell biology. In particular, it may show that T cell activation is not a unified process, and define both intercellular signals and intracellular targets that independently regulate cell cycle progression and differentiation.